1. Field of the Invention
The present invention relates to methods and apparatus for generating a bias current which is supply-independent and temperature-stable.
2. Description of the Related Art
A biasing circuit for producing a bias current which is supply-independent and temperature-stable is critical to the success of most any analog circuit design. For most high-speed analog circuit designs, the bias current must also be maintained over a certain voltage signal swing.
In conventional biasing circuits, reference voltages and currents produced therefrom undesirably fluctuate due to, for example, temperature and integrated circuit (IC) process variations. A conventional biasing current changes not only with the resistor process (which is intentional and desired), but also with the transistor process. Variations from the resistor process are part of the design objective (for a constant swing which is equal to I*R), whereas variations from the transistor process are undesirable.
Advances in CMOS technology are primarily targeted to the design of digital circuits. The modeling of CMOS devices for their analog behavior in low current regions (sub-threshold and near-threshold operation) is inaccurate. Therefore, it becomes necessary to design analog circuits away from these regions to increase the design""s reliability. This requirement translates into utilizing transistors with relatively large Vgs values. As CMOS technology keeps scaling down, however, supply voltages keep getting lower. Thus, the low supply voltage limits the designer""s options.
Accordingly, what are needed are methods and apparatus for generating a bias current which is supply-independent and temperature-stable.
According to the present invention, a biasing circuit for producing a bias current which is supply-independent and temperature-stable includes a first voltage generating circuit, a second voltage generating circuit, a differential amplifier circuit, and a current generating circuit.
The first voltage generating circuit produces a first voltage V1 at its output, and the second voltage generating circuit produces a second voltage V2 different from the first voltage V1 at its output. The first voltage generating circuit includes a first transistor having a first temperature coefficient and a first aspect ratio. The second voltage generating circuit includes a second transistor having a second temperature coefficient that is substantially the same as the first temperature coefficient, and a second aspect ratio that is different from the first aspect ratio.
The differential amplifier circuit has inputs coupled to the outputs of the first and the second voltage generating circuits and produces a reference voltage VREF based on a difference between the first voltage V1 and the second voltage V2. The current generating circuit produces a bias current IREF from the reference voltage VREF. Since the first and second voltages V1 and V2 change with temperature in the same way, and the reference voltage VREF is based on the difference between these voltages, the reference voltage VREF and bias current IREF have temperature coefficients that are zero or nearly zero. Thus, a bias current which is supply-independent and temperature-stable is produced.